1. Field of the Invention
The present invention relates to a spreading code allocation system and method, and in particular to a spreading code allocation system and method, for allocating uniquely spreading codes to base transceiver stations.
2. Description of the Related Art
In a mobile communications system and a licensed personal communication service (PCS) system, an access scheme capable of accommodating as many users as possible In a limited frequency band has been desired. As such an access scheme, there is a frequency division multiple access (heretofore referred to as FDMA) scheme, in which frequency division is conducted so as to allow common use of the frequency band by a plurality of users. There is also a time division multiple access (hereafter referred to as TDMA) scheme, in which division into periodic time slots is conducted.
In addition, the code division multiple access (hereafter referred to as CDMA) scheme capable of having more user channels than the FDMA and TDMA schemes has been standardized by TIA (Telecommunication Industry Association) and EIA (Electronic Industries Association). In the CDMA scheme, the spectrum of a transmission frequency is widened by using a spreading code uniquely allocated to a user to be identified. As a result, a plurality of users can share the same frequency band. At the time of reception, demodulation is conducted by using the same spreading code as used at the time of transmission. Since signals from other users remain to have wide bands, it is possible to easily obtain a received signal which is addressed to its own station and is improved in signal-to-interference power ratio by a spreading gain. Typically as the spreading code, a pseudo-noise (hereafter abbreviated to PN) code functioning as a cyclic code has been used.
PN codes are uniquely allocated to a plurality of users using the same frequency band. Therefore, compared with the FDMA and TDMA schemes, a larger number of users can be accommodated per a predetermined bandwidth. Furthermore, since the transmission frequency band is wide, the frequency selectivity is strong. Therefore, the CDMA scheme provides concealment of information because demodulation cannot be conducted without using the same PN code as that used at the time of transmission.
When adopting the CDMA scheme, for example, in a mobile communications system of a cellular scheme, besides a spreading code allocated to a mobile station, another spreading code Is allocated to the mobile station in some cases. In the mobile communication system of the cellular scheme, there are pluralities of base transceiver stations, and consequently the mobile station needs to identify a base transceiver station with which the mobile station is communicating. Especially near the boundary of an area in which each base transceiver station provides service, It is necessary to change the base transceiver station providing service whenever the mobile station moves from a service area to another service area. In order to identify such a base transceiver station, the different spreading code is needed.
As shown in FIG. 1, consider a mobile communications system including a plurality of base transceiver stations each Identified by a different spreading code. Each of the base transceiver stations (hereafter referred to as BTSs) 231, 232, . . . , 23N provides communication services to a mobile station (hereafter referred to as MS) 24 located within its service area. The first to Nth BTSs 231 through 23N are connected to a base transceiver station controller (hereafter referred to as BSC) 26 via transmission paths 251 through 25N, respectively. The BSC 26 is connected to a mobile switching center (hereafter referred to as MSC) 27. The MSC 27 has a data base 28 associated therewith. The MSC 27 is connected to a public network (not shown) via a public line 29. The MS 24 can communicate with the first through Nth BTSs 231 through 23N by radio signals 301 through 30N, respectively. A radio signal may convey incoming call information to the MS 24, outgoing call information from the MS 24 to a terminal of the opposite party, or communication information.
Each of the 1st through Nth BTSs 231 through 23N has a unique PN code allocated thereto as a spreading code. In the DB 28, PN codes allocated to the respective BTSs are stored in advance. The MSC 27 notifies the respective BTSs of corresponding PN codes via the BSC 26 and the transmission paths 251 through 25N.
The MSC 27 performs call processing between the terminal of the opposite party and the MS 24 managed by the MSC 27. The BSC 26 monitors the BTSs including one communicating with the MS 24 at any time and changes the BTS communicating with the MS 24 to an adjacent BTS depending on movement of the MS 24. When an incoming call addressed to the MS 24 has occurred, the MSC 27 conducts call processing for the BTS serving an area in which the MS 24 exists.
Each BTS periodically sends a pilot PN code of its own BTS, called xe2x80x9cpilot channelxe2x80x9d, to MSs existing in the service area served by the BTS, and transmits information for synchronization acquisition and holding, and clock recovery in each MS. Each MS responds to a BTS sending a pilot channel which is the largest in received intensity. As a result, each BTS can determine whether a MS under search is present in its own service area or not, and notifies the BSC of its result.
For example, it s now assumed that an incoming call to the MS 24 existing in an area served by the Nth BTS 23N has occurred. Since the MS 24 has been recognized to be located within the service area of the Nth BTS 23N by responding to a pilot channel from the Nth BTS 23N, the MSC 27 conducts call setup processing with the MS 24 via the BSC 26 and the Nth BTS 23N. At this time, the MS 24 can recognize from the PN code of the Nth BTS 23N received over the above pilot channel that the call setup is caused by the Nth BTS 23N. In other words, the MS 24 selects and receives only the radio wave from the Nth BTS 23N without being affected by radio waves from the first through (Nxe2x88x921)th BTSs 231 through 23Nxe2x88x921. If the MS 24 has moved into an area served by the third BTS 233, then the movement is periodically monitored by the BSC 26 as a result of MS 24 responding to a pilot channel sent from each BTS. Thereafter, communicating with the MS 24 is performed via the third BTS 233.
As for the synchronization between the BTSs and MSs using PN codes as described above, the time required for the synchronization becomes longer according to the code length of the PN code. To improve the error rate and shorten the time required for synchronization, there has been proposed a technique for generating a PN code such that the code length thereof varies depending on an error rate detected by the BTS in Japanese Patent Application Lad-Open. No 9-181647.
Another technique for shortening the time for synchronization acquisition and the time required for synchronization establishment has been disclosed in. Japanese Patent Application Laid-Open No. 10-56405. More specifically, the time for synchronization acquisition is shortened by reducing the length of a PN code for synchronization used in a pilot channel dedicated for initial synchronization acquisition between a MS and a BTS. The time required for synchronization establishment of the MS with the communication channel transmitted by the BTS is shortened by a phase of an identification code contained in an identification pilot channel agreeing with a phase of an identification PN code contained in a communication channel.
In the case where the FDMA or TDMA scheme is adopted in the mobile communcations system or the licensed PCS system, a limited frequency band is shared by a plurality of users and therefor the frequency allocation to the users is needed. In the system design stage, this frequency allocation is a work greatly influencing the communication quality. If it is attempted to accommodate more users or subscribers in the communications system adopting the FDMA scheme or the TDMA scheme, then it is necessary to divide the limited frequency band as efficiently as possible and to increase the number of BTSs. The increased number of BTSs causes the distance between BTSs to become shorter and the reuse, meaning the repetitive use of the same frequency band, becomes more frequent in the limited frequency band. Therefore, the distance between BTSs using the same frequency band becomes shorter. The larger the number of installed BTSs, the more difficult the frequency allocation to BTSs. In this way, frequency allocation involves a great amount of work.
In the case where the CDMA scheme is adopted, it is necessary to allocate PN codes to respective BTSs in the same way as the frequency allocation in the FDMA scheme or the TDMA scheme. In the conventional mobile communications system shown In FIG. 7, PN codes are manually stored in the DB 28 to allocate them to the BTSs, respectively and the respective BTSs are notified of the PN codes via transmission paths. In the CDMA scheme as well, work of allocating PN codes is needed in the same way as the frequency band allocation in the FDMA scheme or the TDMA scheme. If two different PN codes but close to each other are allocated to adjacent or neighboring BTSs, then there may be a possibility that the PN codes are erroneously detected due to delayed waves generated by multipath caused by configuration of the ground, natural features, and buildings. Therefore, It is necessary to allocate PN codes having a sufficient distance between them to BTSs adjacent to or close to each other.
In the case where it is attempted to accommodate more subscriber users in a communications system adopting the CDMA scheme, more BTSs are installed. Whenever a new BTS is installed, the work of allocating PN codes is needed. For example, in the case where one BTS is newly installed and a PN code is allocated thereto, PN codes which have had a sufficient distance between them, and PN codes must be allocated to all BTSs from the beginning all over again. In this way, the PN code allocation has a problem that it causes a great deal of work.
An object of the present invention is to provide a spreading code allocation method and system which dramatically reduces the amount of work for allocating spreading codes to base stations.
Another object of the present invention is to provide a spreading code allocation method and system which can allocate optimum spreading codes to base stations.
According to the present invention, a method for allocating a plurality of spreading codes to a plurality of base stations is provided. After retrievably storing distances between each of the base stations and other base stations in a memory, the memory is searched for a longest distance of the distances. Based on a necessary spreading code difference and a longest spreading code shift corresponding to the longest distance, a spreading code spacing between spreading codes to be allocated to the base stations is determined. Then spreading codes with the spreading code spacing are sequentially allocated to the base stations.
Preferably, the spreading code spacing determined as follows: comparing the necessary spreading code difference with the longest spreading code shift; and selecting a greater one of the necessary spreading code difference and the longest spreading code shift as the spreading code spacing. Further, the necessary spreading code difference may be obtained by dividing a number of available spreading codes by a number of the base stations.
It is preferable that the spreading codes with the spreading code spacing are allocated to the base stations such that two spreading codes with a smaller spacing are allocated respectively to two base stations which are located at a longer distant from each other.
The spreading code allocation may be performed by the following steps: d-1) selecting a certain base station and a certain spreading code which is allocated to the certain base station; d-2) selecting a base station located at a longest distance from a previously selected base station, excepting a further previously selected base station; d-3) allocating a spreading code to a currently selected base station, wherein a currently allocated spreading code is shifted from a previously allocated spreading code by the spreading code spacing; and d-4) repeating the steps d-2) and d-3) until the spreading codes have been allocated to all the base stations.
According to another aspect of the present invention, in a system for allocating a plurality of spreading codes to of: A plurality of base stations which are connected to a system controller, each of the base stations includes: a test transceiver for transmitting and receiving a predetermined spreading code on a pilot channel to and from other base stations under control of the system controller; a detector for detecting a spreading code shift from the predetermined spreading code received from each of the other base stations; and a transmitter for transmitting the spreading code shift to the system controller. The system controller includes: a database for retrievably storing spreading code shifts received from the base stations; a selector for searching the database for a longest spreading code shift; and an allocation controller for determining a spreading code spacing between spreading codes to be allocated to the base stations based on a necessary spreading code difference and the longest spreading code shift, and for sequentially allocating spreading codes with the spreading code spacing to the base stations.
As described above, according to the present invention, among the distances between a plurality of base stations, a maximum distance is selected and the spacing code spacing is determined based on the maximum distance. Spreading codes are allocated to the base stations with the spreading code spacing.
Therefore, it becomes possible to eliminate the need of manual work of allocating the spreading codes to the base stations in a CDMA system. In addition, it is possible to prevent degradation of the system quality caused by artificial allocation errors which could not be avoided in the manual allocation work.
Further, the spreading codes with the spreading code spacing are allocated to the base stations such that two spreading codes with a smaller spacing are allocated respectively to two base stations which are located at a longer distant from each other. Therefore, optimum allocation of the spreading codes can be achieved, and multiple paths between adjacent base transceiver stations can be eliminated, resulting in improved communication reliability.
In the case where the number of base stations is small, the available spreading code spacing might be greater than the necessary spreading code difference. In such a case, greater allocation spacing can be adopted. In other words, spreading codes separated by at least the necessary spreading code difference are allocated to the base stations. To a base station located farthest from a certain base station, a spreading code nearest to that of the certain base transceiver station is allocated. Therefore, a spreading code having a greater spacing can be allocated to a base station nearer the certain base station. As a result, the possibility of erroneous detection of spreading codes in each base transceiver station can be further lowered. Thus, a communication system having further higher reliability can be realized.